This application pertains to the art of internal structure modification of metal, and more particularly to internal structure modification of metal by ion implantation.
The invention is particularly applicable to internal structure modification of metals that creep at high homologous temperatures and will be described with particular reference thereto. However, it will be appreciated that the invention has broader application and may be advantageously employed in other environments.
Heretofore, metals that tend to creep at high homologous temperatures have been unable to provide and withstand a desired grain shape when exposed to elevated temperatures. Such metals generally tend to recrystallize as temperatures increase, and when this recrystallization occurs, columnar grains spread through the thickness of the metal, causing the metal to become brittle with a likelihood of breaking. The tendency to creep limits the usefulness of many metals, and it has been necessary to dope such metals with certain other elements to promote creep resistance.
Lamp wire is a prominent example of creep resistant metal at very high temperature. Whereas, pure tungsten (and some tungsten alloys) recrystallizes at temperatures not much higher than 1000.degree. C. and creeps under relatively low loads, e.g., its own weight, the potassium-doped tungsten lamp wire does not fully recrystallize until temperatures of greater than approximately 2000.degree. C. are reached. Further, the doped material maintains good creep resistance in lamp wire at high temperatures, e.g., at temperatures greater than 2500.degree. C., and is therefore useful for lamp filaments.
The reasons for this good creep resistance in lamp wire are fairly well understood. The relatively large potassium atoms are virtually insoluble in the tungsten matrix, and because of its large atomic size, potassium does not diffuse readily through the metal lattice. The potassium atoms are trapped in the metal and form bubbles. The bubbles act as pinning points against the movement of dislocations and grain boundaries, and thereby counteract creep and recrystallization. The pinning efficiency is good with a high density of small bubbles, i.e., a large number of bubbles per unit area over which a grain boundary or a dislocation can be pinned. It has been reported that for potassium-doped lamp wire, the pinning strength at 2150.degree. C. is best for bubble sizes smaller than 600 Angstroms.
The fine bubbles in their arrangement in the lamp wire is achieved through extensive thermo-mechanical processing. Initially, the potassium is introduced into the material by doping the tungsten powder. After pressing and sintering, there is considerable mechanical deformation processing by rolling, swaging, and wire drawing. By this mechanical working, the potassium clusters are dispersed into long and very fine ribbons, and the desired fine distribution is achieved. When the material is subjected to high temperatures, the ribbons break up into stringers of fine bubbles, which perform their role of pinning dislocations and grain boundaries. Very large deformations are necessary to effectively produce the fine distribution of the potassium dopant. Such large deformations are achieved during repeated wire drawing, where for example, the reduction is a cross-sectional area and the elongation is as large as by a factor of 1,000 to 1,000,000.
Although conventional lamp filaments provide creep resistance, over 50 processing steps are necessary to accomplish their production. That is, the various stages of mechanical working described above may each occur a plurality of times. In addition, the method of doping lamp filaments as described above does not work for other geometries such as flat sheets because it is not possible to accomplish such extensive plastic deformation in geometries other than those similar to lamp wire.
It is desirable to define a simpler method of making metals, particularly lamp filaments, creep resistant at elevated temperatures. Accordingly, the present invention contemplates a process which overcomes all of the abovedescribed problems and others to provide a method for making metals, particularly lamp filaments, creep resistant at elevated temperatures so as to cause the metal's grain shape to remain intact. These results are achieved by layering in conjunction with ion implantation.